Field of the Invention
The present invention relates to a ventilation structure for a transmission that ventilates a case of a transmission.
Description of Related Art
In a transmission mounted on a vehicle, component parts of a transmission mechanism such as a rotating shaft and a gear are accommodated in a transmission case (hereinafter referred to as “case”). Such transmission is provided with a breather structure for adjusting temperature and pressure inside the case. This breather structure (ventilation structure) is configured to ventilate the case by providing a through hole to the case or installing a pipe (breather pipe) communicating to the outside. For example, see Japanese Patent Application Publication No. 2000-346181 (Patent Document 1).
Inside the case of the transmission, oil (lubricating oil) is accommodated together with the transmission mechanism. In the transmission having a natural circulation type lubricating structure, this oil is scraped up by a rotating body such as a gear and then supplied to each part in the case. For this reason, a larger amount of oil is scraped up particularly when the vehicle is traveling at a high speed. In this case, in the above-described breather structure, high temperature/pressure oil might be ejected from the through hole or the pipe to the outside of the case depending on conditions such as installation positions and directions of the through hole and the pipe.
Therefore, the through hole and the pipe of the breather structure should be configured so as not to eject the oil scraped up in the case. Furthermore, as deteriorated oil is likely to bubble, the through hole and the pipe should be configured also so that oil bubbles disappear.
Under such circumstances, there is a structure for preventing oil from being ejected to the outside. In this structure, the breather pipe is installed in a breather chamber formed in the case for allowing air to escape when internal pressure in the breather chamber exceeds a predetermined value, and oil bubbles are vanished in the breather chamber on the air inlet side of the breather pipe, thereby separating oil from air.
Specifically, there is a structure in which a mesh member for vanishing oil bubbles entering the breather chamber is arranged. For example, see Japanese Patent Application Publication No. 2012-154347 (Patent Document 2). Further, there is a complicated structure (labyrinth structure) having an intermediate wall separating the inside of a cover member disposed on the top of the case into two chambers, thereby receiving oil on the intermediate wall when air mixed with oil splashed in the case enters the cover member to prevent the oil from being ejected from the breather pipe. For example, see Japanese Patent Application Publication No. 2013-108596 (Patent Document 3).
However, the structure for vanishing oil bubbles using the mesh member, as disclosed in Patent Document 2, requires a separate member other than the case, causing such problems that the number of members may increase and that oil smaller than a hole of the mesh member can pass therethrough. Further, the structure disclosed in Patent Document 3 causes a problem that the cover member for forming the labyrinth structure may be so complicated as to increase manufacture cost.